1. Field of the Invention
The present invention relates to an organic electroluminescent display (OELD) device, and more particularly, to an OELD device being capable of displaying an image of uniform brightness and a method of driving the OELD.
2. Background for the Related Art
The liquid crystal display (LCD) device requires a light source therein because it is non-emission type display device. The OELD device is introduced to overcome this disadvantage of an active matrix type liquid crystal display device. In the OELD device, an organic luminescent layer is exited to emit light. The OELD device is driven by a relatively low voltage and has a thin profile.
FIG. 1 is a circuit diagram showing a pixel of an active matrix type OELD device according to the related art. As shown in FIG. 1, two transistors and one capacitor are disposed in the pixel. The OELD device includes a scanning line “S”, a data line “D”, a switching thin film transistor (TFT) “SW”, a capacitor “C”, a driving TFT “DR” and an organic electroluminescent diode “E” on a substrate. For example, each of the switching and driving TFTs “SW” and “DR” may be an NMOS type TFT.
A gate of the switching TFT “SW” is connected to the scanning line, and a source of the switching TFT “SW” is connected to the data line “D”. One end of the capacitor “C” is connected to a drain of the switching TFT “SW”, and a ground voltage “VSS” is applied to the other end of the capacitor “C”.
A source of the driving TFT “DR” is connected to a power line. A driving voltage “VDD” is applied to the drain of the driving TFT “DR” through the power line. A drain of the driving TFT “DR” is connected to an electrode of the organic electroluminescent diode “E”. A gate of the driving TFT “DR” is connected to the drain of the switching TFT “SW”.
A driving principle of the device shown in FIG. 1 is explained with FIG. 2. FIG. 2 is a timing chart of signals in an OELD device according to the related art. When an nth scan signal “S(n)” of a high voltage “Vgh” is applied to the switching TFT “SW” through the scanning line S, the switching TFT “SW” is turned on. The scan signal “S(n)” is generated from a gate driving integrated circuit (IC) (not shown). The high scan signal “Vgh” is a positive voltage. The scan signal “S(n)” is pulsed from a low voltage “Vgl” to the high voltage “Vgh”.
When the switching TFT “SW” is turned on, a data voltage “Vdata” is applied to the capacitor “C” through the data line “D” and the switching TFT “SW” such that an electric charge is charged in the capacitor “C”. Since a channel of the driving TFT “DR” is an NMOS type, the data voltage is positive. Amount of an electric current passes through the channel of the driving TFT “DR” depends upon a potential difference between a charged voltage of the capacitor “C” and the driving voltage “VDD”. Brightness of light from the organic electroluminescent diode “E” is determined by the amount of an electric current passing through the channel of the driving TFT “DR”.
Unfortunately, the driving TFT “DR” in each pixel has a deviation in an electric property such that each pixel has different brightness in the same condition. In a panel using a low temperature poly-silicon type backplane, there is a deviation of an electric property in the driving TFTs “DR” because of an excimer laser annealing process for the low temperature poly-silicon. Accordingly, even if the same voltage is applied to the driving TFT “DR” in each pixel, there are differences in amounts of an electric current passing through the channel of the driving TFT “DR” such that brightness uniformity of images on the OELD device is deteriorated.
On the other hand, in a panel using an amorphous silicon type backplane, there is thermal degradation in the driving TFTs “DR” when the driving TFTs “DR” are driven. Each driving TFT “DR” in the pixels has a difference in the thermal degradation such that brightness uniformity of images on the OELD device is deteriorated.
Referring to FIG. 3, which is a graph showing deviation of an electric current on an organic electroluminescent diode with respect to a voltage on a driving TFT in the related art OELD, an electric current on the organic electroluminescent diode “E” (of FIG. 1) in one pixel is different from that in another pixel because of deviation in an electric property of the driving TFT “DR” (in FIG. 1). As a result, even if each pixel is driven under the same condition, each pixel displays an image having a difference in brightness such that brightness uniformity is deteriorated. For example, afterimages or a stain is generated on the image panel.